For 2-1/2 weeks, the travels and travails of a robotic rover on the most Earth-like planet in the solar system have captured the imagination of millions of people around the world.

Even as the band of youthful engineers at the Jet Propulsion Laboratory here continue to steer their pint-sized geologist over alien soil, National Aeronautics and Space Administration (NASA) planners are laying the groundwork for what could be the most spectacular achievement in the history of human exploration - a manned mission to Mars, perhaps as early as 2007.

The success of the Pathfinder mission is already prompting JPL engineers to begin redesigning coming unmanned missions to look more closely at what must be done to put people on the Red Planet. Preparations range from measuring radiation levels and probing the surface for toxic chemicals to 21st Century Mars: Transforming Wheel Tracks Into Human Treks

testing ways of using carbon and oxygen in the planet's tenuous atmosphere to make rocket fuel.

"We're just beginning to lay out the detailed road map for what needs to be done," says JPL director Edward Stone of NASA's effort.

There is no budget yet for manned Mars hardware. NASA is focused on building the international space station starting in 1998. But a broad Mars mission outline calls for assembling vehicles in low-Earth orbit.

With six launches, NASA officials figure they can put in orbit the components of a mission, including a housing module, outbound rockets, a fuel factory for the return trip, and an Earth-return vehicle.

While circling Earth, each module would be mated to its own nuclear-powered propulsion stage for the four-to six-month-long trip to Mars.

TECHNICALLY, an Earth-orbit launch to Mars appears within grasp. It's less costly than a surface launch and would draw upon 30 years of experience docking with objects in orbit.

Once on the planet, astronauts would set up shop for a 500-day stay, conducting experiments to determine if Mars once hosted life and to provide more data on how suitable the planet is for longer-term human habitation.

But NASA's Mars plan isn't the only working option. Robert Zubrin, an aerospace engineer at Martin Marietta in Denver, holds that within 10 years the United States could put humans on Mars using existing technology - for only about $2 billion.

His timetable calls for using an expendable booster in 2005 to hurtle an unmanned Earth-return vehicle with an empty fuel tank to Mars. Two years later, when Mars again is at its closest point to Earth, NASA would launch a crew and its habitat module. Robotic vehicles fueled with methane made on Mars would support the astronauts' explorations. After a year and a half, the astronauts would head home using fuel made on Mars, leaving their equipment behind for the next crew.

As a priority for the US space program, manned Mars missions have had a checkered history. Early plans called for a 30-year, $300-billion program. In 1989, President Bush proposed a 2019 manned mission.

Last September, however, President Clinton appeared to close the door on sending astronauts to Mars. He threw his support behind unmanned missions, rather than open the federal treasury to NASA's Mars-or-bust crowd. A revised price tag of $100 billion for a manned mission to the Red Planet was still too high, he held, given the uncertain benefits of using humans on the planet. Indeed, NASA's planning process has once again rekindled the debate about whether exploration should be left to humans or sophisticated robots.

But Mr. Clinton left the door ajar, and NASA administrator Daniel Goldin has sent a message to his troops. "They need to figure out a way to get to Mars" within eight years of a go-ahead signal and for less than $25 billion, he says. "They need to resolve the human safety issues, which are many in terms of living and working in space, they have to figure out what the scientific payoffs are, and they have to learn to work with other countries."

"The program will not be brought into the political domain," he says, until those issues are resolved.

The nine unmanned missions to Mars between now and 2005 will help resolve some of those questions, says Norman Hayes, who heads JPL's Mars Exploration Directorate.

RECENTLY, JPL joined forces with NASA's human-exploration group to use JPL's 2001 lander/rover/orbiter package to get at some of the environmental issues astronauts would face, he says. Initially, the lander for 2001 was seen merely as a carrier for a more sophisticated version of Sojourner. Now, researchers will use the lander as a platform for experiments for tests related to radiation exposure and the feasibility of producing elements for fuel from Mars' atmosphere.

Indeed, "virtually everything we do" during the unmanned Mars exploration program will be "directly applicable to what we're going to do when we send humans to Mars," Mr. Hayes says - from precision landings to remotely piloted rovers. "Humans can't do everything by themselves."